The invention relates to a method for synchronizing the operation of hysteresis current controlled parallel connected partial systems.
If high power density and high efficiency are required in a power electronics systems having a parallel connection of partial systems, it is advantageous for the partial systems to operate out-of-phase. By phase shifting the switching action of n partial systems by 2.pi./n, the frequency of the ripple of the total current resulting as the sum of the partial currents is increased by a factor of n. Furthermore, a partial cancellation of current harmonics of the switching frequency is achieved and accordingly the amplitude of the ripple component of the total current is reduced. As compared to n=1 (i.e., to an application of a single system or phase-synchronized operation of the partial systems), for equal amplitude of the ripple of the total current, the switching frequency and/or the switching losses of the partial systems can be reduced and/or the efficiency of the energy conversion can be increased. Alternatively, for equal switching frequency, the inductance of the inductors of the partial systems and the circuitry for the filtering of the switching frequency harmonics of the total current can be reduced and/or the power density of the converter can be increased for equal efficiency.
Based on the prior art for the control of the partial systems, a constant pulse frequency is used because a fixed width of the pulse periods simplifies the synchronization of the partial systems and the definition of a phase displacement of the switching action of the systems. In order to synchronize the n partial systems, the switching action of these systems must be shifted by 2.pi./n. In the simplest case, the switching instants and/or the pulse width modulated control signals of the partial systems are derived from the intersections of the output signal of a current controller which performs a local averaging and dynamic weighting of control error of the total current with phase shifted triangular signals. However, this basically uses a voltage to control the current. Therefore, for guaranteeing a symmetrical current consumption of the partial systems a separate control circuit has to be provided. A further drawback of this averaging method, however, is limited dynamic response of the current control due to the averaging of the control error. This makes it necessary to use a separate overcurrent protection circuit on the system and contributes to an increased complexity of the control circuit.
A direct control, balancing and limitation of the partial currents can be achieved with a low complexity circuit only by application of a current controller, for example, a hysteresis current controller. However, in this case, no direct possibility for synchronization and/or definition of a phase displacement of the switching action of the partial systems exists due to the changing switching frequency of this type of controller.